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Re: [PATCH v6 6/9] xen/riscv: introduce functionality to work with CPU info


  • To: oleksii.kurochko@xxxxxxxxx
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Wed, 11 Sep 2024 14:14:23 +0200
  • Autocrypt: addr=jbeulich@xxxxxxxx; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL
  • Cc: Alistair Francis <alistair.francis@xxxxxxx>, Bob Eshleman <bobbyeshleman@xxxxxxxxx>, Connor Davis <connojdavis@xxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
  • Delivery-date: Wed, 11 Sep 2024 12:14:33 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 11.09.2024 14:05, oleksii.kurochko@xxxxxxxxx wrote:
> On Tue, 2024-09-10 at 12:33 +0200, Jan Beulich wrote:
>> On 02.09.2024 19:01, Oleksii Kurochko wrote:
>>> @@ -72,6 +77,16 @@ FUNC(reset_stack)
>>>          ret
>>>  END(reset_stack)
>>>  
>>> +/* void setup_tp(unsigned int xen_cpuid); */
>>> +FUNC(setup_tp)
>>> +        la      tp, pcpu_info
>>> +        li      t0, PCPU_INFO_SIZE
>>> +        mul     t1, a0, t0
>>> +        add     tp, tp, t1
>>> +
>>> +        ret
>>> +END(setup_tp)
>>
>> I take it this is going to run (i.e. also for secondary CPUs) ahead
>> of
>> Xen being able to handle any kind of exception (on the given CPU)?
> Yes, I am using it for secondary CPUs and Xen are handling exceptions (
> on the given CPU ) fine.

Yet that wasn't my question. Note in particular the use of "ahead of".

>>  If
>> so, all is fine here. If not, transiently pointing tp at CPU0's space
>> is a possible problem.
> I haven't had any problem with that at the moment.
> 
> Do you think that it will be better to use DECLARE_PER_CPU() with
> updating of setup_tp() instead of pcpu_info[] when SMP will be
> introduced?
> What kind of problems should I take into account?

If exceptions can be handled by Xen already when entering this function,
then the exception handler would need to be setting up tp for itself. If
not, it would use whatever the interrupted context used (or what is
brought into context by hardware while delivering the exception). If I
assumed that tp in principle doesn't need setting up when handling
exceptions (sorry, haven't read up enough yet about how guest -> host
switches work for RISC-V), and if further exceptions can already be
handled upon entering setup_tp(), then keeping tp properly invalid until
it can be set to its correct value will make it easier to diagnose
problems than when - like you do - transiently setting tp to CPU0's
value (and hence risking corruption of its state).

Jan



 


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